Along with rapid reduction in size and weight and diversification of cellular phones, portable appliances, and mobile information terminals for civil uses, with respect to batteries for their power sources, it has been required to develop secondary batteries which are small in size and light in weight, have high energy density, and can repeatedly be charged and discharged for a long duration. Especially, as compared with lead batteries and nickel-cadmium batteries using aqueous electrolyte solutions, nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries as secondary batteries satisfying these requirements are most highly expected and various investigations have actively been made.
A nonaqueous electrolyte secondary battery includes a positive electrode, a negative electrode, and a nonaqueous electrolyte and as a positive active material have been investigated various compounds such as titanium disulfide, vanadium pentoxide, molybdenum trioxide, as well as compounds defined by a general formula LixMO2 (wherein, M is one or more transition metals) such as lithium cobalt composite oxides, lithium nickel composite oxides, and spinel type lithium manganese oxides. Especially, lithium cobalt composite oxides, lithium nickel composite oxides, and spinel type manganese composite oxides can actualize batteries having high discharge voltage by being used as a positive electrode since they are capable of charge-discharge at extremely high (noble) potential of 4V (vs Li/Li+) or higher.
As a negative active material of a nonaqueous electrolyte secondary battery, various kinds of metal lithium, lithium alloys, carbon materials cable of absorbing and releasing lithium, and the like have been investigated, and especially, if a carbon material is used, it is advantageous since batteries excellent in charge-discharge cycle performance can be obtained and safety is very high.
As an electrolyte of a nonaqueous electrolyte secondary battery have been employed electrolyte solutions obtained generally by dissolving supporting salts such as LiPF6 and LiBF4 in mixed solvents of high dielectric solvents such as ethylene carbonate and propylene carbonate with low viscosity solvents such as dimethyl carbonate and diethyl carbonate.
In recent years, demand for nonaqueous electrolyte secondary batteries as a power source for transport bodies such as hybrid cars has been increased and also capability of charge and discharge at high electric current to correspond to rapid charge-discharge, that is, retention of high input and output performance for a long duration has been required besides the high energy density conventionally regarded to be important.
With respect to a method for suppressing an increase of the inner resistance of a nonaqueous electrolyte secondary battery during long time use, no method has been made available yet and it is required to further enhance the charge-discharge cycle performance by improving an active material, a current collection structure, a separator, and a nonaqueous solution and improving a method of adding and mixing additives in an electrolyte solution.
Patent Document 1 discloses that a battery containing an electrolyte solution to which a cyclic sulfuric acid ester is added shows small capacity deterioration caused along with the course of charge-discharge cycle. Patent Document 2 discloses that a battery provided with a nonaqueous electrolyte solution containing 1,3-propene sultone shows low self discharge and is suppressed from deterioration of load performance and resistance and that the amount of a gas generated in the battery is decreased.
Patent Document 3 contains a description in claim 1, “a nonaqueous electrolyte secondary battery having a nonaqueous electrolyte containing at least one compound selected from sultone compounds, cyclic sulfate esters, and vinylene carbonates and at least one derivative selected from alkylbenzene derivatives having tertiary carbon neighboring to a phenyl group, cycloalkylbenzene derivatives, and biphenyl derivatives. However, there is no specific description of batteries using both of a sultone compound and a cyclic sulfate ester.
None of Patent Documents 1 to 3 describe a battery of which an increase of inner resistance is suppressed, as well as capacity retention ratio is high and expansion is small even in a case of long time use by employing an unsaturated sultone and a cyclic sulfate ester in combination and specifying a positive active material composition.
Patent Document 1: Japanese Patent Application Laid-Open No. 10-189042
Patent Document 2: Japanese Patent Application Laid-Open No. 2002-329528
Patent Document 3: Japanese Patent Application Laid-Open No. 2003-308875